1. Field of the Invention
The present disclosure relates to a voltage switching circuit, a secondary battery protection circuit incorporating the voltage switching circuit, and a battery pack for a secondary battery, such as a lithium-ion battery used for electronic equipment, incorporating the secondary battery protection circuit.
2. Description of the Background Art
Recently, small hand-held electronic devices, such as mobile phones, digital cameras, laptop computers, and personal digital assistances (PDA) and the like, have become very popular. Typically, secondary batteries (rechargeable batteries) consisting of a high-capacity battery such as a nickel metal hydride battery, a lithium-ion battery, or a lithium polymer battery have come to be widely used in those portable electronic devices. A drawback of such a configuration is that accidental short-circuiting or overcharging can send a high current through the battery and connecting circuitry, which may cause the battery to overheat or even destroy the electronic device. In addition, overcharging the lithium-ion secondary battery causes metallic lithium to be deposited and may pose an accident. Further, over-discharge degrades the battery.
To avoid the above-described problems, a protection switch is provided in a charge-discharge route between the secondary battery and the electronic device. When the protection switch detects that a charging voltage exceeds a predetermined voltage (overcharge) or that a discharging voltage falls below a predetermined voltage (over-discharge), the protection switch is turned off, which prevents further overcharge and over-discharge. In order to prevent short-circuiting and abnormal charging of the battery pack caused by excess current, the protection switch changes the secondary battery to a charging inhibit state and a discharging inhibit state when the charging current or the discharging current exceeds a certain level.
In a conventional charge-discharge protection circuit for the secondary battery (secondary battery protection circuit), a negative terminal of the battery pack and a negative terminal of a charger connection (charger negative power supply terminal) are set as excess-current detection terminals, a charge-discharge control metal oxide semiconductor (MOSFET) is used as a resistor, a current flowing through the MOSFET is converted to voltage using an ON resistance of the MOSFET, and the voltage is input to the secondary battery protection circuit, which controls the excess current.
However, heating of the charge-discharge control MOSFET caused by the flow of current and differences in the ON resistance of the MOSFET from one manufacturer to the next hinders improvement in excess current detection accuracy.
In an effort to counteract the above-described problem, JP-2005-168159-A proposes a secondary battery protection integrated circuit (IC) that improves the excess current detection accuracy by using a current-detection resistor to convert current to voltage.
The secondary battery protection IC includes two charge-discharge control MOSFET. When the excess current is detected with the current-detection resistor, the secondary battery protection IC outputs a charge-discharge control signal after excess current detection and turns the corresponding charge-discharge control MOSFET off, which restrains the current.
At this time, since one of the charge-discharge control MOSFET is turned off, the flow of current is stopped, the voltage converted from the current by the current-detection resistor and input is dropped, and the secondary battery protection IC immediately returns to a state in which the current flows. Therefore, in order to prevent return to such a state once the excess current is detected, a latch operation is required of the internal circuitry of the protection IC. In addition, to return from the latched state after excess current detection to a chargeable-dischargeable state, certain conditions must be taken into account.